Positron emission tomography (PET) has been known as one method of detecting cancer by taking cross-sectional images of human body. PET is a device for determining position of cancer cells, by detecting radiation emitted from positron-emissive nuclear species incorporated into the cancer cells. In PET, radiation is converted by a scintillator into short-wavelength light. The light after the conversion is further converted by a photo-electric conversion element into an electric signal, so that the radiation is detected by analyzing the electric signal.
In position determination of cancer cells, it is important to improve accuracy of detection timing of radiation. One of erroneous factors in the detection timing of radiation may be ascribable to variation in slope of the electric signal depending on pulse height, and this may result in an error of incidence start time of radiation. One technique of clearing the causal factor is known as constant fraction timing. As is known from description typically in Non-Patent Document 1, the technique is designed to detect the incidence start time of radiation, based on time corresponded to zero-cross point of a signal produced by summing an attenuated signal of an input signal and an inverted signal of the input signal. Non-Patent Document 1 also describes other techniques analogous to the constant fraction timing.